In accordance with the latest functional developments in computer processors and application programs, it has become necessary to efficiently expand the connection capacity of computers for various kinds of peripheral devices. Conventional external ports of computer systems have already come up to limits of the peripheral devices with various functions and interface forms. For those demands, a new bus interface system, referred to as the “universal serial bus” (hereinafter, referred to as “USB”), has been developed by major computer and telephone networks companies, such as Intel, Microsoft, Compaq, NEC, and so on, in order to provide practical applications adaptable to various interfacing needs. The USB now has become a new standard for interfacing between computers and peripheral devices. The USB standard V1.0 was first defined on Jan. 15, 1996, and was revised to USB V1.1 on Jul. 28, 1998.
The USB device can be assigned to a multiplicity of peripheral devices numbering, for example, 127. One peripheral device includes 16 end points. Therefore, it is possible to grant 16 functions at maximum in one peripheral device. There are various peripheral devices connectable to USB devices, such as telephones, MODEMs, printers, scanners, game pads, microphones, digital speakers, styluses, joysticks, mice, monitors, or digital cameras.
Interface cables connecting a host computer to a peripheral device, or connecting between peripheral devices, are composed of a power source voltage (VDD) line, a ground voltage line (VSS), and a pair of data signal lines (D+, D−). The data signals must be leveled in the CMOS voltage range of 3.3V as an example.
The USB V1.1 also defines optional speed modes of full-speed (or high-speed) and low-speed. The high-speed mode is operable at 12 Mbps (mega bits per second) while the low-speed mode is 1.4 Mbps. In the low-speed mode, since bus occupation rate can be eight times that in the high-speed mode, data transmission conditions are negatively affected for high frequency devices when too many devices are connected thereto or short cycles are dominant in processing data. Therefore, the low-speed mode is adaptable to devices in need of low cost and low power consumption, or to devices with smaller amounts to data transmission, such as a mouse or a keyboard.
The pair of data signals (D+, D−) is a complementary pair in which one is a high level when the other is a low level. The data signal is designed to rise up to a high level or to fall down to a low level, with a predetermined slope. According to the USB standard, a crossover voltage, i.e., a voltage point where one data signal slope rising to a high level from a low level meets the other data signal slope falling to a low level from a high level, should be within the range between 1.3V and 2.0V.
Transmission data in the form of NRZI (Non-Return-to-Zero-Invert) are converted into bus-specific data signals (D+, D−) modulated by a transceiver to be adaptable to USB cables. A crossover voltage of the bus-specific data signals is sensitive to threshold voltages of transistors constructed in the transceiver. For instance, assuming that a distribution profile of threshold voltages is 0.9V±0.1V, the threshold voltages ranges from 0.8V at minimum to 1.0V at maximum. As a result, there occur differences between a rising time and a falling time, which causes variation of crossover voltage of the bus-specific data signals. If there is a jitter of time difference due to a difference between transition speeds of the data signals at a USB transmission stage, it is hard to obtain correct data at a USB receiving stage.